A number of references show the use of endless belts to fix toner images to paper, transparency stock or the like; see, for example, U.S. Pat. No. 3,948,215; European Applications 0301585 and 0295901. Typically, in these fixing processes a combination of heat and pressure is applied between an endless belt and a pressure roller with the toner image on a receiving sheet facing the endless belt. The toner image is left in contact with the endless belt until the image is cooled below the glass transition temperature of the toner, at which point the receiving sheet can be separated without offset.
U.S. Pat. No. 4,968,578, Light et al, issued Nov. 6, 1990; U.S. Pat. No. 4,927,727, Rimai et al, issued May 22, 1990; and U.S. Pat. No. 5,021,835, Johnson, issued Jun. 4, 1991, all described a heat-assisted toner image transfer method. Two or more single color images are transferred in registration from an image member to a receiving sheet having a heat softenable thermoplastic outer layer, by heating the receiving sheet to an elevated temperature. The temperature of the receiving sheet is sufficiently above the softening point of the toner and of the layer that the toner sticks to the receiving sheet. This method is particularly useful in transferring extremely small, dry toner particles, for example, toner particles having a mean particle diameter of 8 microns or less.
Especially in transferring a series of single color toner images to a form multicolor toner image, the layers of toner pile up above the level of the receiving sheet even when substantial pressure is used in transfer. This results in an unacceptable relief image corresponding generally to the optical density of the image. U.S. Pat. No. 5,023,038 to Aslam et al, issued Jun. 11, 1991; U.S. Pat. No. 5,089,363 to Rimai et al, describe a method of fixing such toner images to a receiving sheet which receiving sheet has an outer heat-softenable thermoplastic layer. The relief image is substantially reduced, the image is more permanently fixed and gloss can be increased by bringing the image into contact with a ferrotyping surface, for example, a hard surface of a metal belt, under conditions of heat and pressure which cause the image to be further embedded in the thermoplastic layer. In addition to being hard, the ferrotyping surface has good release. For example, it can be made of nickel, stainless steel or other metals, with or without surface treating with silicones or the like. The ferrotyping surface can be textured to provide a matte or other textured finish to the image, or it can be smooth to enhance its gloss.
In designing a continuous production image forming apparatus the ferrotyping surface is formed on a web. The web is usually in the form of an endless belt, but it is also known to be quite long and have supply and take-up rolls for continuous operation.
As in the earlier cited belt fixing apparatus, the toner image and the thermoplastic layer are left in contact with the belt until they are cooled below their glass transition temperatures before separation. Preventing offset by cooling in contact with the web eliminates the need for offset preventing liquids which have a degrading affect on a high quality image.
A problem in using a web system, especially an endless belt system in a productive image forming apparatus is associated with the time required for the belt and image to cool while maintained in contact. If the fixing device is moved at a speed below the speed of the transfer station to allow cooling, then the mismatch of speeds between the transfer station and the fixing device must be accommodated. In general, this requires either a full-frame distance in the in-track direction between the transfer station or drum and the fixing device, or a loop or other mechanism absorbing the difference in speeds.
A number of references suggest actively cooling the belt and receiving sheet combination to reduce the necessary size of the belt required. For example, "Belt Fusing Device", Research Disclosure, July 1990, Page 559, suggests a heat pump device which removes heat from the portion of the belt leading away from the pressure applying members and transfers the heat to the portion of the belt approaching the pressure applying members. In U.S. patent application Ser. No. 07/754,489 filed Sep. 3, 1991 to J. P. Swapceinski et al shows a heat transferring roller positioned between portions of an endless belt fuser to conduct heat from the cooling portions to a portion about to be heated. U.S. Pat. Nos. 4,780,742 and 3,948,215 suggest air cooling the belt and receiving sheet after it leaves the pressure applying members. See also U.S. Pat. No. 5,012,291; 3,356,831; 3,948,215 for other cooling and heating devices for belt fusers.
U.S. patent application Ser. No. 07/783,475 to Johnson and Merle, entitled IMAGE FORMING APPARATUS INCLUDING TONER IMAGE FIXING DEVICE USING FUSING SHEETS, filed Oct. 28, 1991, suggests using a finite or a cut fusing sheet instead of an endless belt which fusing sheet can form a sandwich with the receiving sheet which can be moved much slower or not at all during the cooling process after leaving a pair of pressure members that can be allowed to run at full machine speed.